Assembly having variable capacitance

ABSTRACT

The present invention relates to an assembly of variable capacitance as well as to a method of operating the assembly. In the assembly, a variable capacitor is formed by a variable coverage or a variable distance of at least one first ( 4 ) and one second electrically conductive region ( 5 ). The first electrically conductive region ( 4 ) is configured on or in a substrate ( 1 ) and said second electrically conductive region ( 5 ) is configured on or in an actuator element ( 3 ) of a first micro-mechanical actuator ( 2 ). The actuator ( 2 ) is disposed on the substrate ( 1 ) in such a way that it can perform a movement of the actuator element ( 3 ) with the second region ( 5 ) along a surface of the substrate ( 1 ) at different positions relative to the first region ( 4 ), at which positions the second region ( 5 ) overlaps the first region ( 4 ) at least partly. Moreover, holding means ( 6, 10, 11 ) are provided which are capable of pulling or pushing the actuator element ( 3 ) in the different positions towards the substrate ( 1 ) or a mechanical stop ( 13 ) on the substrate ( 1 ), and of holding it in these positions. 
     The inventive assembly serves to implement a variable capacitance that presents a high stability in resistance to outside influences according to its respective setting.

FIELD OF THE INVENTION

The present invention relates to an assembly of variable capacitance inaccordance with the introductory clause of Patent claim 1, as well as toa method of setting a predeterminable capacitance by application of thisassembly.

The principle field of application of the present invention is withinthe domain of high-frequency technology, particularly in applications incommunication technology. In this field, there is an ever-increasingdemand for settable high-frequency capacitance levels at a maximum ofquality achievable. So far, semiconductor components, so-calledvaractors, have been used to this end, which, however, reach Q-factorlevels between 20 and 40 at maximum. Specifically for the application inmobile telephony or cellular phones, however, Q-factors of at least 100are desirable. These high Q-factors in a resonant circuit can beachieved, at present, only with the use of micro-mechanical capacitors.

PRIOR ART

Capacitors manufactured by micro-engineering means are mechanicallymobile elements that permit a variation of the distance or theoverlapping or coverage degree of two capacitor plates for settingdifferent capacitance levels. When such mobile elements are used areaction of the component to exterior accelerations cannot be completelyprecluded, however. To this adds that in the high-frequency range at thefrequencies from 0.8 to 2 GHz, which are specifically employed incellular telephones and with the intended low attenuation levels, mainlymetal materials are used as basic material for the mobile elements.Their specific density is definitely higher than the density in silicon,for instance, so that the mass of the mobile element is additionallyincreased by these materials. Low driving voltage levels and theresulting low driving forces for the mobile elements, in combinationwith the desired positioning distances of several 10 μm, requirecomparatively soft suspending systems.

On the other hand, a settable capacitor for mobile application should beas stable as possible so that neither thermal nor mechanical influencesfrom the outside cannot lead to a drift in capacitance. This resistance,particularly to accelerations, cannot be achieved with theafore-described properties of the known micro-mechanical assemblies.

The present invention is therefore based on the problem of providing anassembly of variable capacitance as well as a method of operating thisassembly, which ensure a high stability of the respective capacitanceset in resistance to outside influences.

BRIEF DESCRIPTION OF THE INVENTION

The problem of the invention is solved by the assembly according toclaim 1 as well as by the method according to claim 18. Expedientembodiments of the assembly as well as of the method are the subjectmatters of the dependent Claims.

In the inventive assembly, the variable capacitance is constituted by avariable coverage or a variable distance of at least one first and onesecond electrically conductive region. In the context of the presentpatent application, the term “coverage” denotes an at least partialmutual overlapping or coverage of the two regions, seen in a viewingdirection substantially orthogonal on or parallel to the surface of thesubstrate of the assembly.

The first electrically conductive region is configured here on or in thesubstrate while the second electrically conductive region is configuredon or in an actuator element of a first micro-mechanical actuator. Bothregions are preferably constituted by plane layers or plate-shapedelements and are substantially parallel to each other whenever thecapacitance is set. However, this is not a definitely requiredprerequisite for the function of the assembly.

The first micro-mechanical actuator is so configured and disposed on thesubstrate that it is capable of performing a movement of the actuatorelement with the second region along the surface of the substrate atdifferent positions relative to the first region, where the secondregion overlaps, at least partly, the first region. The first region ispreferably disposed in or below the substrate surface and in parallelwith the latter. The first region, however, may also extend in the formof a separate structure in a direction orthogonal on the substratesurface.

In the different positions which the actuator element may take, hencedifferent distances and/or degrees of overlapping coverage prevailbetween the first and second regions, which gives rise to a differentcapacitance level. In accordance with the present invention, moreoverholding means are provided on the substrate, which are capable ofpulling or pushing the actuator element in the different positionstowards the substrate or a mechanical stop on the substrate, and ofholding it in this position. This holding function of the presentassembly prevents a variation of the respective positions set and henceof the capacitance level set in the event of outside influences actingupon it.

The fixing of the actuator element relative to the substrate or thefirst region, respectively, can be ensured by both an electrostaticholding force and a further holding element producing a purelymechanical action upon the actuator element.

The holding means for the implementation of an electrostatic holdingfunction can be achieved here in a very simple manner by theconfiguration of further electrically conductive regions on or in theactuator element and on or in the substrate, which are opposed to eachother in the different positions to be set, so that the actuator elementwill be pulled towards the substrate surface due to the application of adifferential voltage between these further electrically conductiveregions. As a matter of fact, either an insulating layer must beconfigured over the additional electrically conductive region on theactuator element or on the substrate, or the respective regions are heldby spacer or stops on the substrate or on the actuator element at adistance in order to avoid a short circuit. This applies also to thefirst and second electrically conductive regions that constitute thevariable capacitance. These regions, too, must not come into directcontact with each other when the holding position is realised.

The holding means may also be implemented by a thermo-mechanical microactuator. This micro actuator is so configured and disposed relative tothe first micro-mechanical actuator that in response to a thermalexcitation, it will be deflected in a substantially orthogonal directionon the surface of the substrate and that a first section of the actuatorelement of the first micro-mechanical actuator in the differentpositions to be set or that can be set up reaches up to a positionunderneath a second section of the thermo-mechanical actuator—if thelatter is in a deflected state. When the thermo-mechanical actuator isswitched off the actuator element of the first micro-mechanical actuatoris then clamped between the first section of the thermo-mechanicalactuator and the substrate. Due to this clamping effect, a holdingfunction can be expediently implemented, which does not require thatenergy be supplied during the holding function. For release of thisholding position, the thermo-mechanical actuator is, in its turn, heatedso that it will expand or will be deflected, respectively, and hencereleases again the first micro-mechanical actuator.

In addition, the respective sections of the two actuators, which aresuperimposed on each other, may present corresponding structures thatpermit a mutual engagement or mutual hooking at the respective holdingpositions. This ensures a particularly stable holding position.

The first micro-mechanical actuator may be configured, for instance, aselectrostatic or thermo-mechanical actuator. With appropriatesuitability, micro actuators operating on other driving principles can,equally be employed, of course. Electrostatic micro actuators are,however, particularly well suitable for the application in anetwork-independent device such as a cellular telephone, due to theirlow energy consumption. Moreover, an electrostatic operation permitshigh-speed switching in the range of 100 μs.

The structure of suitable micro-mechanical actuators is common to thoseskilled in the art, which are appropriate for the application in theinventive assembly. The common methods of microstructure technology canbe applied for the manufacture of such micro actuators and of thepresent assembly. For the manufacture of the variable capacitors,specifically those methods come into question, which operate either onthe basis of polysilicon or on methods for the realisation of themechanical components proper. Both manufacturing techniques are part ofthe field of superficial micro mechanics.

In operation of the inventive assembly, the first micro-mechanicalactuator is deflected in the envisaged manner and when the desiredposition or capacitance, respectively, is reached the holding means arecontrolled to maintain this position. The respective actual position ofthe actuator element may preferably be realised by measuring anappropriate reference capacitance (or differential capacitance,respectively) on the substrate. The measurement of this referencecapacitance during the deflection enables a very high resolution or avery precise setting of the variable capacitance. The referencecapacitance can be constituted by additional small capacitors that maybe adjacent to the variable capacitance proper (high-frequencycapacitance). These additional capacitors may be employed asposition-sensitive sensors. The actual target capacitance, however, isreached only when the holding function is activated because this holdingfunction varies the distance between the first and second electricallyconductive regions again, at least in the preferred embodiment of theassembly. In the operation of the present assembly in a closed-looparrangement, with integration of the additional reference capacitor orreference capacitors, respectively, hence the desired capacitance of theassembly can be set with a very high precision.

In an expedient operating mode, the first micro-mechanical actuator isperiodically controlled at its natural frequency. This can be realisedin a simple manner particularly when this actuator is driven in anelectrostatic manner. The second electrically conductive regionperiodically sweeps over the first electrically conductive region and,when the desired position is reached, it is fixed by controlling theholding means so that the desired capacitance value is maintained. Dueto the periodic control, wide deflections of the actuator element can bereached with comparatively low driving voltages. Moreover, in thisvariant of the operating mode of the assembly, the above-describedclosed-loop assembly is preferably employed for determining theposition.

The operating range of the inventive assembly can be extended by theprovision that a number of additional switchable capacitors are disposedon the substrate. These switchable complementary capacitors consist, forinstance, of invariable capacitors that can be additionally connectedvia appropriate high-frequency switches of the variable capacitance. Theadditional switchable discrete capacitance elements may also beimplemented in their capacitors in a binary arrangement. Due to thecombination of such a capacitor network with the variable capacitanceproper, it is possible to set a wide range of capacitance levels.

In another alternative embodiment, several ones among the inventiveassemblies are disposed on a substrate whose variable capacitanceelements are connected in parallel.

The inventive assembly as well as the associated method will bedescribed again in the following by embodiments, with reference to thedrawings, in an exemplary form, without a restriction of the generalinventive idea. In the drawings:

FIG. 1 illustrates a first embodiment of the configuration of aninventive assembly with electrostatic drive means;

FIG. 2 a 3D view of the example shown in FIG. 1;

FIG. 3 shows a second embodiment of a configuration of the inventiveassembly with thermo-mechanical drive means; and

FIG. 4 is a view of a further example of an embodiment of the inventiveassembly.

WAYS OF REALISING THE INVENTION

FIG. 1 shows a first example of a conceivable embodiment of theinventive assembly. In this embodiment, electrostatically operatedactuators are used to vary the capacitance and to realise the holdingfunction. The first micro-mechanical actuator 2—referred to as lateralactuator—generates a stroke of its actuator element 3 in parallel withthe surface of the substrate 1. With this lateral actuator 2, hence thesetting of the overlapping or coverage of the first electricallyconductive region 4 and of the second electrically conductive region 5is achieved to form the capacitance. The first electrically conductiveregion 4 is here disposed on an insulating layer 12 on the surface ofthe substrate 1. The second electrically conductive region 5 is formedon the underside of the actuator element 3.

The holding function is achieved by a second micro-mechanical actuatorthat is formed by two mutually opposed electrically conductive regions10, 11 on the surface of the substrate 1 or on the underside of theactuator element 3, respectively. This holding means, which will bereferred to as vertical actuator, pulls the actuator element 3 down ontothe substrate 1 after it has reached its nominal position, whilst itfixes the position of the actuator element 3 or of the second region 5,respectively, relative to the first region 4, in this manner. Thedirections of movement of the vertical actuator and the lateral actuatorare roughly indicated by the arrows in the illustration. The firstactuator element 3 is attracted onto the substrate 1 by means of thevertical actuator 6 takes place up to corresponding stop structures onthe substrate, which are not illustrated in the Figure. These insulatedsupports prevent an electrical short-circuit of the variable capacitanceformed by the two electrically conductive regions 4, 5.

FIG. 2 illustrates a three-dimensional view of an inventive assembly asillustrated in FIG. 1. In this Figure, too, the substrate 1 with thefirst electrically conductive region 4 can be recognised again, on whichthe lateral actuator 2 is disposed with an appropriate actuator element3. The actuator element 3 is formed here by beam-shaped elementscarrying a plate-like element 5 on its end as second electricallyconductive region. Apart from this second region 5, electricallyconductive regions 11 are provided that cooperate with electricallyconductive regions 10 located on the substrate 1 to constitute thevertical actuator 6. The lateral electrostatic drive of the firstactuator 2 is implemented via capacitors 8 with parallel plates, whichare disposed in a comb-like arrangement. Thus, a movement along they-axis is achieved, which is indicated by two arrows. In order toachieve a maximum of lateral stroke possible by means of theelectrostatic drive system, the actuator element 3 of the, presentexample is equipped with a mechanical converter that increases theachievable stroke by a factor of roughly 5 to 6. This conversion isachieved and simultaneously amplified by two bending joints 9 thatconvert the movement along the y-direction into a movement along thex-direction. The resulting movement along the x-direction is equallyindicated by an arrow. On account of this mechanical conversion, it ispossible to achieve strokes of 20 to 30 μm at electrostatic drivingvoltages of less than 12 V.

The leads 7 to the actuators 2, 6 or of the variable capacitor 4, 5,respectively, are schematically indicated in the Figure.

FIG. 3 illustrates a second embodiment of a configuration of theinventive assembly. In this example, thermo-mechanical actuators areemployed not only for the generation of the lateral deflection but alsofor the implementation of the holding function. In the Figure, thesubstrate 1 is with the first electrically conductive region 4 can berecognised again. Moreover, the horizontal thermo-mechanical actuator 2as well as the vertical thermo-mechanical actuator 6 are illustrated onthe substrate. The first thermo-mechanical actuator 2 moves the upperplate 5 of the high-frequency capacitor in a horizontal direction overthe substrate 1. Due to the two capacitor plates 4, 5 overlapping, thedifferent capacitance levels can be set. The lateral actuator 2 consistshere of an arrangement of bending elements 3 whose height exceeds theirwidth. When heated by means of an integrated heater below these bendingelements 3, the entire actuator or the entire bending element moves inparallel with the substrate surface. The plate-like second electricallyconductive region 5 is disposed on the actuator element 3 under afurther plate-like cantilever arm 14 that extends in the direction ofmovement.

The vertical actuator 5 consists of a plate-shaped element whose widthis substantially wider than its thickness and which is clamped on bothends on the substrate 1. Due to this configuration, the plate whenheated bends in a direction orthogonal on the surface of the substrate1. The heating is performed by current flowing through heatingconductors or heating layers integrated into the plate.

When a defined ON/OFF switching sequence is observed for the twoactuators 2, 6 it is possible to hold the lateral actuator 2 in itsposition due to the lowering movement of the vertical actuator 6. To seta desired capacitance value, the lateral actuator 2 is thereforeactivated and displaced into a defined position so that the requireddegree of coverage of the two capacitor surfaces 4, 5 is adjusted. Withthis deflection, the cantilever arm 14 of the lateral actuator is pusheduntil it is located under a cantilever arm 15 of the vertical actuator 6that must equally be in its deflected position by that point of time.When the vertical actuator 6 is switched off it is lowered onto thecantilever arm 14 of the lateral actuator 2, clamping it on thesubstrate or on appropriate supporting areas 13 on the substrate,respectively. The supporting areas 13 can be seen in the upper leftsection of the enlarged view in this Figure. With the application ofthis mechanical clamping action the disadvantage of the high energydemand is eliminated, which is present in thermo-mechanical actuators.In this assembly, the electrical energy must be supplied only during thecomparatively short switching phases.

The use of thermo-mechanical actuators presents, on the other hand, theparticular advantage that it permits the achievement of comparativelywide lateral movements and hence comparatively wide capacitance settingranges. The Figures also shows schematically indicated structures thatpermit hooking of the cantilever arm 14 of the lateral actuator 2 withthe cantilever arm 15 of the vertical actuator 6.

FIG. 4 finally illustrates another embodiment of the design of aninventive assembly wherein a cover chip 16 is mounted on the substrate 1to protect the assembly. The assembly as such, with the lateral actuator2, the actuator element 3 and the capacitor 4, 5, corresponds to theassembly of FIGS. 1 and 2. In this example, the feed lines leading tothe high-frequency capacitor 4, 5 are designed as micro strip-type lines17 in order to achieve the lowest attenuation possible on the lead. Tothis end, the interior of the cover 16, which is shown again in a bottomview in the upper part of the illustration, is coated with anappropriate metal layer 19. This metal layer 19 may be manufactured, forexample, of copper, gold or silver. The cover may be mounted either onthe wafer level or on the chip level when the assembly is manufactured.In any case, the cover should, however, be mounted before the wafer isseparated, in order to prevent soiling of the assembly. The metal layer19 is surrounded by a solder glass frame 18 in this example.

LIST OF REFERENCE NUMERALS

1 substrate

2 first or lateral micro actuator

3 actuator element

4 first electrically conductive region

5 second electrically conductive region

6 second or vertical micro actuator

7 leads

8 parallel-plate capacitors

9 bending joints

10 third electrically conductive region

11 fourth electrically conductive region

12 insulating layer

13 supporting area

14 cantilever arm on the lateral actuator

15 cantilever arm on the vertical actuator

16 cover chip

17 micro strip-type line

18 solder glass frame

19 metal coating

What is claimed is:
 1. Assembly of variable capacitance comprising avariable coverage or a variable distance of at least one firstelectrically conductive area and at least one second electricallyconductive area, with said at least one first electrically conductivearea being configured on or in a substrate and with said at least onesecond electrically conductive area being on or in an actuator elementof a first micro-mechanical actuator that is disposed on said substratein such a way that said first micro-mechanical actuator is capable ofperforming, by means of a first drive system, movement of said actuatorelement with said at least one second electrically conductive area alonga surface of said substrate at different positions relative to said atleast one first electrically conductive area at which positions said atleast one second electrically conductive area overlaps said at least onefirst electrically conductive area at least partly; and holding meanscapable, by means of a second drive system independent of said firstdrive system, of pulling or pushing said actuator element in saiddifferent positions towards said substrate or a mechanical stop on saidsubstrate and of holding said actuator element in such positions. 2.Assembly according to claim 1, wherein said holding means includes atleast one third electrically conductive area on or in said substrate andat least one fourth electrically conductive area on or in said actuatorelement that overlap each other in the different positions of saidactuator element at least partly and are capable of being subjected to adifference in electrical voltage.
 3. Assembly according to claim 1,wherein said holding means includes a second micro-mechanical actuator.4. Assembly according to claim 3, wherein said second micro-mechanicalactuator is a thermo-mechanical actuator that is so configured anddisposed relative to said first micro-mechanical actuator that saidsecond micro-mechanical actuator is deflected in a directionsubstantially orthogonal on the surface of said substrate in response tothermal excitation and that a first section of said actuator element ofsaid first micro-mechanical actuator in the different positions reachesup to a zone underneath a second section of said second micro-mechanicalactuator in a deflected state.
 5. Assembly according to claim 4, whereinsaid second mirco-mechanical actuator includes one or severalbeam-shaped elements that are clamped on both sides of said substrate.6. Assembly according to claim 4 wherein, said first section of saidactuator element of said first micro-mechanical actuator is a plate-likecantilever arm that extends along a direction of movement of saidactuator element.
 7. Assembly according to claim 4, wherein said secondsection of said second micro-mechanical actuator is a plate-likecantilever arm that extends in a direction opposite to a direction ofmovement of said actuator element of said first micro-mechanicalactuator.
 8. Assembly according to claim 4, wherein said first sectionand said second section are constructed and arranged such that theyengage each other when the thermal excitation of said secondmicro-mechanical actuator is terminated while said actuator element ofsaid first micro-mechanical actuator is in said different positions. 9.Assembly according to claim 1, wherein said first micro-mechanicalactuator is an electrostatic micro actuator.
 10. Assembly according toclaim 9, wherein said first micro-mechanical actuator comprises a drivesystem with parallel-plate capacitors in a comb-like arrangement. 11.Assembly according to claim 9, wherein said actuator element comprisesbending joints to enlarge a stroke of movement that can be achieved withsaid first micro-mechanical actuator.
 12. Assembly according to claim 1,wherein said first micro-mechanical actuator is a thermo-mechanicalmicro actuator.
 13. Assembly according to claim 12, wherein said firstmicro-mechanical actuator includes one or several beam-shaped elementsthat are clamped on both sides of said substrate.
 14. Assembly accordingto claim 1, wherein said at least one first electrically conductive areaand said at least one second electrically conductive area are plate-likeareas or elements.
 15. Assembly according to claim 1, wherein electricalleads to said at least one first electrically conductive area and saidat least one second electrically conductive area are micro strip-typelines and that a protective cover for said micro-mechanical actuator onsaid substrate comprises a metal coating on its inner surface. 16.Assembly according to claim 1 further comprising invariable capacitorsconfigured on said substrate, which are adapted to be switchedseparately via switching elements parallel with said variable capacitorformed by said at least one first electrically conductive area and saidat least one second electrically conductive area.
 17. Assembly accordingto one of claims 1 to 16, wherein one or more of said assembly arepresent and are connected in parallel.
 18. Method of setting apredeterminable capacitance by means of an assembly according to one ofclaims 1 to 16, wherein said actuator element of said firstmicro-mechanical actuator is moved to a position in which said at leastone second electrically conductive area presents a spacing from and anat least partial coverage with said at least one first electricallyconductive area, which correspond to said predeterminable capacitance,and said holding means are controlled for holding at this position. 19.Method according to claim 18, wherein a respective actual position ofsaid actuator element is detected by a measurement of a referencecapacitance between one or several regions provided on said actuatorelement and on said substrate.
 20. Method according to claim 18, whereinsaid first micro-mechanical actuator is operated at its resonancefrequency.